Near misses raise rocky questions

A hero always saves the Earth from impending asteroids in Hollywood movies. In reality, would we have any warning, asks David Adam in London.

January 14 could have been a bad day for George Bush. As the President was preparing to announce America's return to the moon in a speech at NASA headquarters, he was almost asked to deliver a very different message: that the Earth could be struck by a devastating asteroid within 24 hours.

Astronomers have revealed that during a "nine-hour crisis" the night before Mr Bush's speech, they believed there was a one in four chance an asteroid would hit the planet in 36 hours. Had it not been for a break in the clouds that allowed an amateur astronomer to give the all-clear, the scientists say they were on the verge of calling the White House.

David Morrison, an asteroid and comet impact hazard expert at NASA's Ames Research Centre, said a preliminary analysis suggested that the asteroid could hit Earth in less than two days.

"And if a possibility of an impact in two days existed, what should we do about notifying governments or the public?" he asked.

The scientists could only say they thought the 30-metre asteroid would strike somewhere in the northern hemisphere; at that size it would have exploded well before reaching the ground, though it could have killed thousands if it broke up over a big city.

The object - named 2004 AS1 - turned out to be around 500 metres wide, and passed Earth at a safe range of about 12 million kilometres: some 32 times the distance between the Earth and the moon.

Details of the events of January 13 were revealed this week by veteran sky-watcher Clark Chapman at a planetary protection conference in California.

Astronomers confirmed that the asteroid could be on a collision course, but the telescopes that could see the relevant part of the sky were obscured by cloud. Several hours after the scare began, an amateur astronomer, Brian Warner in Colorado, saw through a gap in the clouds and confirmed that the area of sky was empty.

The episode, the latest in a series of false alarms, pointed to the disquieting prospect of an asteroid showing up suddenly on Earth's doorstep with no time for Hollywood heroics.

No guidelines exist for who should have been informed and when and what emergency measures should have been taken if the threat had been real.

"I would not have been comfortable with being quiet through the next morning," said Chapman, from the Southwest Research Institute in Boulder, Colorado, who was involved in discussions about the meteor that night. "I think the public should be informed of that high a probability, of that big an event, occurring."

Donald Yeomans, head of NASA's Near Earth Objects program office, said he would not have raised an alarm until a second set of observations confirmed the collision path.

He said he hoped the episode would prompt the setting of guidelines for future warnings.